web_server: Return early if no clients connected

[jesserockz]

What does this implement/fix?

Even when no clients are connected to the /events endpoint, the JSON is still being generated. This adds an early return to not do anything if there are no clients.

The size_t count() function exists already in the Arduino library, and this PR adds it to the esp-idf implementation.

[19:14:20][V][json:038]: Attempting to allocate 512 bytes for JSON serialization
[19:14:20][V][json:058]: Size after shrink 64 bytes

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @dentra, mind taking a look at this pull request as it has been labeled with an integration (web_server_idf) you are listed as a code owner for? Thanks!
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